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(I explained this syntax in the Filling a DataTable section earlier in this chapter.) What actually happens is that the DataAdapter reads all the records before the ones you re really interested in, and then it discards them. So this approach is OK for small resultsets, but you should never use it for tables containing more than a few hundred rows. You have to roll up your sleeves and start writing some smart SQL code to implement a better paging mechanism. Let s start by having a look at the following graph, which depicts a small Publishers table of just 12 records, divided into three pages of four rows each. The resultset is sorted on the PubId numeric key.
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PubId 1 3 Page 1 7 12 19 28 Page 2 33 51 66 75 Page 3 78 81 Publishers table
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Getting the first page is easy, thanks to the TOP clause offered by both Access s SQL and T-SQL dialects:
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SELECT TOP 4 * FROM Publishers ORDER BY PubId
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Moving to the next page is also simple. Say that you re currently positioned on a page in the middle of the table, such as the one that appears in dark gray in the preceding diagram, and you want to read the next four rows. These rows are the first four records whose key value is higher than the value of the key at the last record in the current page:
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-- 51 is the value of the key at the last row in the current page. SELECT TOP 4 * FROM Publishers WHERE Pubs > 51 ORDER BY PubId
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Getting the last page is also simple: we just need to retrieve rows in reverse order and select the first four rows of the result. To keep things simple, let s assume that the resultset contains an integer multiple of the page size:
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SELECT TOP 4 * FROM Publishers ORDER BY PubId DESC
The problem here is that we get the result in reverse order and therefore must reverse the rows. We might do this after we load the rows into the DataTable, but it s better to have the database engine do the job for us so that we can bind the resultset directly to a DataGrid control. So we need to run the preceding query as a subquery of another SELECT command that puts all rows in the correct order:
SELECT * FROM Publishers WHERE PubId IN (SELECT TOP 4 PubId FROM Publishers ORDER BY PubId DESC) ORDER BY PubId
Implementing the Previous button is slightly more complex because we want the four records that come immediately before the key value in the first row of the current page, as in this code snippet:
-- 19 is the value of the key at the first row in the current page. SELECT TOP 4 * FROM Publishers WHERE PubId < 19 ORDER BY PubId DESC
Again, this query returns a resultset whose rows are in reverse order, so we need to run the query as a subquery of another SELECT that puts everything right again:
SELECT * FROM Publishers WHERE PubId IN (SELECT TOP 4 PubId FROM Publishers WHERE PubId < 19 ORDER BY PubId DESC) ORDER BY ISBN
We re now able to implement the First, Previous, Next, and Last buttons in our Win dows Form or Web Form. We can also add a Goto button that displays the N th page. For example, showing the fifth page would mean reading the first 20 rows and extract ing the last four rows of the result:
SELECT TOP 4 * FROM Publishers WHERE PubId IN (SELECT TOP 20 PubId FROM Publishers ORDER BY PubId) ORDER BY PubId DESC
This query returns the rows in reverse order, so we must run it as a subquery of another query that re-sorts the result in the correct order:
SELECT * FROM Publishers WHERE PubId IN (SELECT TOP 4 PubId FROM Publishers WHERE PubId IN (SELECT TOP 20 PubId FROM Publishers ORDER BY PubId) ORDER BY PubId DESC) ORDER BY PubId
I told you that implementing paging isn t a trivial task, remember Anyway, at this point creating the application has become just a matter of running the right queries against
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